1. Field of the Invention
The present invention relates to gas conveying duct joints and, more particularly, to joints for plastic ducts.
2. Description of the Prior Art
Typically, ducts for central furnace and air conditioning units and the like are formed of sheet-metal. However, there are several disadvantages to using sheet-metal ducts. First, sheet-metal is susceptible to rust and corrosion. Second, sheet-metal conducts electricity. Third, sheet-metal does not take paint easily. In addition, sheet-metal has a relatively low insulation value (R value) and is relatively heavy.
Furthermore, the joining of sheet metal ducts in a secure and air-tight manner has been a constant problem in the prior art. Numerous U.S. patents disclose complex sheet metal duct joining brackets. See, for example, U.S. Pat. Nos. 2,260,221; 3,805,845; 3,836,181; 4,046,409; 4,294,476; and 4,304,423. Thus, there has been a strong need in the art for a new type of air duct, as well as a method of joining same, obviating the need for such complicated joining brackets.
Furthermore, when installing air ducts in buildings, for example, the ducts are typically cut to odd lengths in order to accommodate the particular room configuration and dimensions. Thus, there has been a strong need in the art for a duct joining system which can accommodate ducts of standard length as well as ducts of odd sized lengths.
Recognizing the deficiencies of metal ducts used for corrosive fume handling systems, Harvel Plastic, Inc. of Easton, Pa. has designed a exhaust system using plastic ducts. The Harvel system utilizes ducts composed of polyvinyl chloride (PVC) and having a round or rectangular cross-section. The plastic ducts are fitted with PVC gaskets and thermally welded joints. Flexible connections are welded, cemented or clamped with stainless steel bands. However, this type of joint between plastic ducts fails to allow for thermal expansion and contraction of the duct. Moreover, thermally welded joints frequently have insufficient reinforcement and strength. Also, these ducts require the use of cumbersome tools such as welding guns for their installation.